1. Field of the Invention
The present invention relates generally to vapor-compression refrigeration systems and more particularly to refrigeration systems having a refrigeration compressor and liquid refrigerant pump with a liquid pre-cooling heat exchanger to suppress flash gas bubbles at the pump inlet port.
2. Description of the Prior Art
In the United States and other countries, refrigeration systems are important for providing cooling in buildings and automobiles and in enabling safe and inexpensive food storage and transportation. The importance and number of refrigeration systems are continuing to grow with further industrialization and urbanization and as the growing population increases the demand for housing, automobiles, refrigerators, and similar products. The main purpose of a refrigeration system is to cool an enclosed space or medium to a lower temperature and to discharge absorbed heat into a higher temperature medium, such as air outside the enclosed space or other medium. To accomplish this type of cooling, it is necessary to do work on a refrigerant, such as ammonia or a halocarbon, to xe2x80x9cpumpxe2x80x9d heat absorbed from the space being cooled into the higher temperature space.
In this regard, the most widely used refrigeration systems are compressor-driven (i.e., vapor-compression) refrigeration systems in which a compressor performs the work on the refrigerant. In typical vapor-compression refrigeration systems, cooling is achieved by passing a refrigerant through the following four basic components: an evaporator, a compressor, a condenser, and an expansion device or a valve. During operation, high pressure liquid refrigerant from the condenser passes through the expansion device, which reduces the pressure and the temperature of the liquid refrigerant. This low pressure, low temperature liquid refrigerant flows through the evaporator and evaporates as the refrigerant absorbs heat from air or liquids passing through or in heat exchange contact with the evaporator. The gaseous refrigerant is then drawn out of the evaporator by the compressor, which pumps the gaseous refrigerant to the condenser by raising the refrigerant pressure, and thus the refrigerant temperature. The gaseous refrigerant condenses to a liquid in the condenser as it gives up heat to a cooling medium that is passed through or in heat exchange contact with the condenser. The liquid refrigerant then flows to the expansion device where the cooling cycle begins again.
The efficiency or coefficient of performance (COP) of the vapor-compression refrigeration cycle can be measured as the ratio of heat absorbed in the lower temperature area to the amount of work that is put into the system, which, for the above system, would be the amount of energy required to operate the compressor.
While effective in providing cooling, a continuing concern with vapor-compression refrigeration systems has been the cost to initially purchase, to maintain, and to operate these refrigeration systems. A key component of the operating costs is the cost of energy for operating or driving the compressor. The cost of energy is generally the cost of electricity, because compressors are often driven by an electric motor, although internal combustion engines, steam turbines, and other driving devices may also be employed. To control or reduce energy costs, it is desirable to maintain and, more preferably, to increase the efficiency of the refrigeration system to obtain a desired amount of cooling at lower energy input levels, i.e., less work performed by the compressor. By increasing the efficiency of the refrigeration system, maintenance costs may also be improved as components, such as the compressor, are operated at conditions and at capacities more closely matching the conditions for which the components of the refrigeration system were designed and selected. With the widespread use of these refrigeration systems, refrigeration components and refrigeration systems having enhanced efficiency would be highly desirable in reducing the operating and maintenance cost of each system as well as resulting in a very large worldwide savings in operating (i.e., energy savings) and maintenance costs.
One method of increasing refrigeration system efficiency is to maintain the cooling levels or heat absorption levels while reducing the amount of work input to the refrigeration system by the compressor and other components. Industrial refrigeration systems often use separate, stand alone liquid refrigerant pumps to reduce the amount of overall energy used to perform cooling. Some commercial systems also employ liquid refrigerant pumps, primarily to overcome piping pressure drop. These designs use liquid pumps to increase refrigerant pressure above that available from the vapor compressor or to circulate liquid refrigerant for a variety of applications. Liquid refrigerant pumps are infrequently used in non-industrial applications because designers are unfamiliar with liquid pumping methods and are reluctant to increase initial system cost, complexity, and physical size.
The high cost of the stand alone, liquid pump is the need for a durable unit that is sealable to prevent refrigerant leakage. The air conditioning and refrigeration industry is highly competitive on initial or installation costs and skeptical of non-mainstream technology, which often requires customizing of existing refrigeration systems and support equipment. Therefore, widespread adoption of liquid pumps for new refrigeration system applications and for retrofit of existing refrigeration systems will probably not occur until a lower cost implementation of this energy saving concept is discovered.
There is still a need for refrigeration system methods and apparatus which improve the operating efficiency of refrigeration systems employing a wide variety of refrigerants and equipment, such as compressors and condensers, at an acceptable initial cost and with a technical design that is acceptable to the refrigeration industry, i.e., technology that is perceived as mainstream for the refrigeration industry and that is readily useful in typical refrigeration applications.
The present invention seeks to reduce the cost, complexity, reliability, and physical space requirements for liquid refrigerant pumping by combining the several components into an integrated unit that can be used in a wide range of sizes. The invention uses one electric motor to drive a vapor compressor and liquid refrigerant pump. The several components are enclosed within a sealed pressure housing.
Pumping any liquid near its saturation temperature is always problematic, and this is particularly true with refrigerant. When liquid refrigerant exits a condenser it is at or slightly below saturated condition. When it enters the lower pressure region of a pump, the refrigerant moves above saturated conditions and gas bubbles will form in the liquid stream. These bubbles result in pump cavitation. Cavitation reduces pump effectiveness and damages pump components. Typical refrigerant pumps must maintain a minimum Net Positive Suction Head (NPSH) to prevent cavitation. Often achieving the required NPSH is impractical on a refrigeration system. However, the present invention solves this problem by incorporating a pre-cooling heat exchanger to sufficiently subcool liquid before it enters the pump to prevent gas bubble formation and cavitation.
Refrigerant flow through a system will vary under different operating conditions. A fixed speed motor will cause a driven pump to induce the same amount of energy into the liquid stream. As such, a pump will not always provide the exact desired pressure increase. To resolve this problem, the present invention has a pressure relief device added to the pump discharge to recycle excess cooled liquid flow into the pump suction. This prevents excess pressure increase where such increases would be undesirable. Furthermore, by relieving excess liquid flow back through the pre-cooling heat exchanger, liquid is further cooled until it approaches the temperature of the cold vapor stream. The resulting heat exchange lowers liquid enthalpy, increasing its ability to perform useful work in the refrigeration system. Furthermore, refrigeration evaporators typically operate at 10-15xc2x0 F. of superheat to ensure dry gas entering the vapor compressor. In the present invention the heat exchange between liquid and vapor warms the vapor stream before the vapor enters the vapor compressor portion of the invention. This allows increased evaporator effectiveness by increasing the amount of liquid in the evaporator and reducing or eliminating superheat in the vapor stream between evaporator and compressor. The pre-cooler heat exchanger provides the necessary superheat to ensure dry vapor entering the vapor compressor portion of the invention.
The present invention is a compressor-pump unit for use in a vapor-compression refrigeration system. The compressor-pump unit comprises a driving device including a rotatable shaft. A compressor is coupled with a first portion of the shaft for compressing gaseous refrigerant within the vapor-compression refrigeration system. A liquid pump is coupled with a second portion of the shaft for receiving liquid refrigerant having a first pressure and for discharging the received liquid refrigerant at a second pressure with the second pressure being higher than the first pressure by a predetermined amount such that the discharged liquid refrigerant is subcooled. A pre-cooling circuit is connected to the liquid pump with the pre-cooling circuit being exposed to the gaseous refrigerant whereby the gaseous refrigerant absorbs heat from the liquid refrigerant, prior to the liquid refrigerant entering the liquid pump.
The present invention additionally includes a combined refrigeration unit having a refrigeration compressor for compressing gaseous refrigerant and a liquid refrigerant pump for receiving and discharging a liquid refrigerant. The combined refrigeration unit comprises a liquid pre-cooling heat exchanger connected to the liquid refrigerant pump and exposed to the gaseous refrigerant with the gaseous refrigerant absorbing heat from the liquid refrigerant within the liquid pre-cooling heat exchanger, prior to the liquid refrigerant entering the liquid pump.
The present invention further includes a method of enhancing the operational efficiency of a refrigeration system having a refrigeration compressor for compressing gaseous refrigerant and a liquid refrigerant pump for receiving and discharging a liquid refrigerant. The method comprises the steps of exposing the liquid refrigerant to the gaseous refrigerant prior to the liquid refrigerant entering the liquid pump.